Ohhhhhh yeaaaahhhh! I am from Argentina, and I am a fan of the foldarap. The customs burocracy have delayed my built :'(. These improvement are very cool... Thanks for the entire design!!
How It works that "ball plunger spring"?

I notice in this case it rotates around the centre of the piece whereas in previous versions it hinged to the side causing the Z axis to rest higher than the pivot when folded. Did you make an intermediary design where it hinged from the side but used the ball springs?
I was working on a foldable printer design a few years back but stopped however I have revamped the idea and been inspired by your wonderful work. However I’m making it at a larger scale and was hoping to use a direct Extruder, which is throwing up an issue or two that I think would be best fixed by offsetting the hinge.
Also, how stable is this setup? Have you ever considered using more Vslot pivoted from the top of Z that can latch on to the Y to create a triangle and add stabilisation? It can then be unlatched to fold up and remain compact.

Alas for you I did not made an intermediary design, because now (center hinge) the balls fit the two positions (folded/unfolded).

And I think I see what you mean about the triangulation, as I'm considering to rebuild the Mendel sized FoldaRap, that also came in mind to add a bar.
But for the original size, the spacing between the balls and the spring force, etc. are just right